Micromachined (MEMS) gyroscopes have become established as useful commercial items. Generally speaking, a MEMS gyroscope incorporates two high-performing MEMS devices, specifically a self-tuned resonator in the drive axis and a micro-acceleration sensor in the sensing axis. Gyroscope performance is very sensitive to such things as manufacturing variations, errors in packaging, driving, linear acceleration, and temperature, among other things. Basic principles of operation of angular-rate sensing gyroscopes are well understood and described in the prior art (e.g., Geen, J. et al., New iMEMS Angular-Rate-Sensing Gyroscope, Analog Devices, Inc., Analog Dialog 37-03 (2003), available at http://www.analog.com/library/analogDialogue/archives/37-03/gyro.html, which is hereby incorporated herein by reference in its entirety).
The principles of vibratory sensing angular rate gyroscopes with discrete masses are long-established (see, for example, Lyman, U.S. Pat. No. 2,309,853 and Lyman, U.S. Pat. No. 2,513,340, each of which is hereby incorporated herein by reference in its entirety). Generally speaking, a vibratory rate gyroscope works by oscillating a proof mass (also referred to herein as a “shuttle” or “resonator”). The oscillation is generated with a periodic force applied to a spring-mass-damper system at the resonant frequency. Operating at resonance allows the oscillation amplitude to be large relative to the force applied. When the gyroscope is rotated, Coriolis acceleration is generated on the oscillating proof mass in a direction orthogonal to both the driven oscillation and the rotation. The magnitude of Coriolis acceleration is proportional to both the velocity of the oscillating proof mass and the rotation rate. The resulting Coriolis acceleration can be measured by sensing the deflections of the proof mass. The electrical and mechanical structures used to sense such deflections of the proof mass are referred to generally as the accelerometer.
Many MEMS gyroscopes employ balanced comb drives of the type described generally in Tang, U.S. Pat. No. 5,025,346, which is hereby incorporated herein by reference in its entirety. General use of a micromachined layer above a semiconductor substrate with Coriolis sensing perpendicular to that substrate is described generally in Zabler, U.S. Pat. No. 5,275,047, which is hereby incorporated herein by reference in its entirety. Exemplary MEMS gyroscopes are described in Bernstein, U.S. Pat. No. 5,349,855; Dunn, U.S. Pat. No. 5,359,893; Geen, U.S. Pat. No. 5,635,640; Geen, U.S. Pat. No. 5,869,760; Zerbini, U.S. Pat. No. 6,370,954; and Geen U.S. Pat. No. 6,837,107, each of which is hereby incorporated herein by reference in its entirety. The latter four patents employ rotationally vibrated mass(es).
Micromachined gyroscopes using comb drives are extensively used in many applications, such as for automotive safety, camera stabilization, gesture-based controllers and many other low-cost applications. Typically, such micromachined gyroscopes include one or more resonant masses (which may be referred to as shuttles or dither resonators) that may be configured to resonate in a vibratory mode, such as in a linear (back-and-forth) vibratory mode or a rotational vibratory mode. Each shuttle is typically driven by oppositely-acting sets of comb drives, where one set of comb drives is used to pull the shuttle in one direction and the other set of comb drives is used to pull the shuttle in the other direction. An alternating drive signal is applied to the sets of shuttles to alternately pull the shuttle in one direction and then the other direction, and so forth, until the shuttle reaches a desired equilibrium. In micromachined gyroscopes that have multiple shuttles, the shuttles may be mechanically or electrically coupled to resonate substantially in synchronization with one another.
A major element in the cost of manufacture of such gyroscopes is measurement and correction of both sensitivity to angular rate and output in the absence of angular rate (null bias), especially as functions of temperature. Specifically, gyroscopes are typically calibrated as part of the manufacturing process to address the following manufacturing tolerance issues, which are particularly troublesome with linearly vibrated gyroscopes:
a) Variability of critical dimensions (CD) within a structure causes mismatch of shuttle frequencies and is the primary cause of Q variation and consequently rate sensitivity error. It also limits the useable capping pressure, which in turn limits the noise that can be achieved as reasonable yield.
b) Variability of fill gas pressure and composition during capping also causes variation in Q and variation of sensitivity with temperature. It interacts with (a) in a complex way.
c) Variability of CD within a structure causes mismatch of drive finger lateral forces, which is a major cause of null bias error.
d) In the presence of an effective quadrature servo, the main cause of null variability with packaging, user mounting and temperature is die stress which also mismatches lateral drive gaps.
Calibration typically involves characterizing output signals from the gyroscope in response to various controlled physical movements imparted to the gyroscope across a range of temperatures. Such calibration is time-consuming and complex, and requires specialized calibration equipment. Thus, calibration is major cost contributor.
In addition to calibration and the costs associated with calibration, gyroscopes often include structural and electronic components that are used to mitigate null bias and angular rate sensitivity errors (e.g., trim electrodes and related circuitry), and these components can add cost and complexity to the gyroscope.
For example, variation of sensitivity with Q against temperature can be achieved open-loop (as, for example, described by Geen et al., Single-Chip Surface Micromachined Integrated Gyroscope With 50°/h Allan Deviation, IEEE Journal of Solid-State Circuits, Vol. 37, No. 12, pp. 1860-1866, December 2002, which is hereby incorporated herein by reference in its entirety) or by sensing the velocity of the shuttle as a compensating variable (as in the commercial gyro ADXRS453) or by stabilizing the velocity of the shuttle with control of the drive. This last technique has been used since the earliest vibratory gyroscopes (for example, Meredith, U.S. Pat. No. 2,455,939, which is hereby incorporated herein by reference in its entirety, e.g., FIG. 5B and col.7 line 50) and continues to attract refined modern implementations (for example, Platt, U.S. Pat. No. 6,718,823, which is hereby incorporated herein by reference in its entirety). However, none of these devolve the absolute sensitivity from a plethora of electronics gains, and therefore calibration is still needed.
The absolute sensitivity can be freed of electronics gain and realized as a function of Q only, using the methods disclosed in U.S. Pat. No. 6,470,748, which is hereby incorporated herein by reference in its entirety. However, temperature variation still needs compensation.
Thermal and mounting variation of null bias can be mitigated using a variety of methods for mechanically isolating the die (as for example, described by Harney et al., U.S. Pat. No. 6,768,196), but these involve additional fabrication and packaging expense.
The lateral drive force can be discerned separately from Coriolis force by modulating the drive, filtering the modulation from the Coriolis signal and nulling it with a servomechanism and electro-mechanical actuator as explained in US patent application publication 2011/0041609, which is hereby incorporated herein by reference in its entirety. However, the filters required are dependent on Q, which makes them difficult to implement in practice without adversely affecting dynamic errors or incurring some calibration technique.